The invention relates to a multiple video server system in which video data digitized through a CATV network, an exclusive-use line network, or the like of a constant bit rate is transmitted to a user terminal from a video server constructed by an ATM or LAN network of a variable bit rate. More particularly, the invention relates to a multiple video server system in which an ATM network which can transfer data at a high speed at a variable bit rate is emulated to a constant bit rate and the data is transmitted.
In association with the development of the recent semiconductor technique, video and audio compressing technique such as MPEG1, MPEG2, or the like, and a network technique, a server system which corresponds to a multimedia and provides digital video data has been put into practical use as a product and its offer has been being started. Video and audio data is provided for the multimedia server system through various kinds of transmission path networks such as LAN (Local Area Network), ISDN, ATM (Asynchronous Transfer Mode), CATV, and the like.
Among such video server networks, in the case where a network on the server side is constructed by using the network of a variable bit rate (VBR), for example, the LAN network or ATM network as a nucleus and video and audio data is distributed to the user by an exclusive-use line in the outside of the server or a transmission path network of a constant bit rate (CBR) such as a CATV network, or the like, it is required that the data is converted to the data of a constant bit rate on the transmission side of the server and is transmitted to the CATV network. Therefore, it is necessary to convert the transmission data to the data of a constant bit rate in an exit portion of the network of a variable bit rate locating on the server side and to transmit to the CATV network.
In the conventional video server system, a data buffer of a large capacity is provided at an entrance of the CATV network which needs the conversion into a constant bit rate and video data is sent from the server to the data buffer through the ATM network of a variable bit rate. When the data buffer is likely to overflow, a signal to stop the data transmission is sent to the server through the ATM network. Therefore, a flow control such that the data is read out from the data buffer in accordance with a reference clock of a constant bit rate without causing an overflow of the data buffer and is transmitted to the CATV system is executed. In the video server system, on the other hand, there is a case where what is called a multi-address transmission such that the video/audio data of the same program is sent from the server to a plurality of user terminals is executed. In this case, a (1:n) communication such that there is one transmitting source and there are (n) reception destinations is executed. However, since the number of transmission data to a plurality of partners is equal to one, the flow control of the (1:1) communication cannot be performed. Therefore, it is necessary to execute a one-way communication such that the transmission according to a constant bit rate is performed on the server transmission side and no response signal is returned at the entrance of the CATV network. As a data transmission of a constant bit rate, in an ITU-T (International Union-Telecommunication Standardization Sector), an ATM forum, or the like, there has been proposed a control method whereby the contents of the transmission data are read out on a network card and recognized and transmitted in accordance with the time information written in the internal data of the transmission. According to the method of converting to the constant bit rate, it is necessary to perform the network transmission by installing an exclusive-use CPU of a high processing ability as a network card. There is, consequently, a problem such that as a present commercially available general network card, for example, an ATM card of the ATM network cannot be used. Since it is necessary to perform multiple processes to a plurality of partners, an exclusive-use CPU of a high processing ability has to be installed to the network card. There is a problem of an increase in costs.